Distinct Dominant Intraseasonal Oscillation Modes Modulate South China's Persistent Extreme Precipitation in Rainy Season

Abstract This study investigates mechanisms by which distinct intraseasonal oscillation modes drive regional persistent extreme precipitation events over South China during the rainy season. Based on periodic characteristics of 54 such events, two dominant modes are identified: 12–20‐day (type‐1) and 25–50‐day (type‐2) oscillations. Although comparable in precipitation magnitudes, these two types are governed by distinct mechanisms. For type‐1, an upper‐level anomalous cyclone accelerates midlatitude westerlies and induces an anomalous anticyclone over SC, which creates favorable local thermodynamic‐dynamic conditions for extreme precipitation in conjunction with 12–20‐day convection systems. For type‐2, the southeastward propagation of 25–50‐day Rossby wave trains intensifies an anticyclone over South Asia, which strengthens northwesterlies into SC. This enhanced flow facilitates cold air intrusion and its interaction with warm moist air, thereby enhancing barotropic instability. Diagnostic analysis reveals that moisture convergence dominates thermodynamic enhancement in type‐1, whereas warm air advection by intraseasonal horizontal winds strengthens low‐level upward motion in type‐2.